Chemical Reactions and Chemical Equations Chemical Equations and Stoichiometry Chemical Reactions in Solution Determining the Limiting Reactant Other Practical Matters in Reaction Stoichiometry Focus On Industrial Chemistry
Chemical Reaction Nitrogen monoxide + oxygen → nitrogen dioxide Step 1: Write the reaction using chemical symbols. Step 2: Balance the chemical equation.
Balancing Equation Strategy Balance elements that occur in only one compound on each side first. Balance free elements last. Balance unchanged polyatomics (or other groups of atoms) as groups.
Fractional coefficients are acceptable and can be cleared at the end by multiplication. Slide 7 of 29
EXAMPLE 4-2 Writing and Balancing an Equation: The Combustion of a Carbon-Hydrogen-Oxygen Compound. Liquid triethylene glycol, C6H14O4, is used a a solvent and plasticizer for vinyl and polyurethane plastics. Write a balanced chemical equation for its complete combustion.
EXAMPLE 4-3 Relating the Numbers of Moles of Reactant and Product How many moles of H2O are produced by burning 2.72 mol H2 in an excess of O2? Write the Chemical Equation: Balance the Chemical Equation:
2 H2 + O2 → 2 H2O
Use the stoichiometric factor or mole ratio in an equation: nH2O = 2.72 mol H2 2 mol H2O = 2.72 mol H2O 2 mol H2 Slide 11 of 29
EXAMPLE 4-6 Additional Conversion Factors in a Stoichiometric Calculation: Volume, Density, and Percent Composition. An alloy used in aircraft structures consists of 93.7% Al and 6.3% Cu by mass. The alloy has a density of 2.85 g/cm3. A 0.691 cm3 piece of the alloy reacts with an excess of HCl(aq). If we assume that all the Al but none of the Cu reacts with HCl(aq), what is the mass of H2 obtained?
EXAMPLE 4-6 2 Al + 6 HCl → 2 AlCl3 + 3 H2 Plan the strategy: cm3
g alloy cm3
g Al g alloy
mol Al g Al
mol H2 mol Al
g H2 mol H2
alloy → g alloy → g Al → mole Al → mol H2 → g H2 We need 5 conversion factors!
Write the Equation and Calculate: 97.3 g Al mH2 = 0.691 cm3 alloy 2.85 g alloy 100 g alloy 1 cm3 1 mol Al 3 mol H2 2.016 g H2 = 0.207 g H 2 2 mol Al 26.98 g Al 1 mol H2 Slide 14 of 29
4-3 Chemical Reactions in Solution Close contact between atoms, ions and molecules necessary for a reaction to occur. Solvent We will usually use aqueous (aq) solution.
Molarity Amount of solute (mol solute) Molarity (M) = Volume of solution (L) If 0.440 mol of urea is dissolved in enough water to make 1.000 L of solution the concentration is: curea
EXAMPLE 4-9 Calculating the Mass of Solute in a Solution of Known Molarity. We want to prepare exactly 0.2500 L (250 mL) of an 0.250 M K2CrO4 solution in water. What mass of K2CrO4 should we use? mol L
Plan strategy:
g mol
Volume → moles → mass
We need 2 conversion factors! Write equation and calculate: mK2CrO4 = 0.2500 L 0.250 mol 194.02 g = 12.1 g 1.00 mol 1.00 L
EXAMPLE 4-10 Preparing a solution by dilution A particular analytical chemistry procedure requires 0.0100 M K2CrO4. What volume of 0.250 M K2CrO4 should we use to prepare 0.250 L of 0.0100 M K2CrO4?
Plan strategy:
Vi Mf = Mi Vf
Vi = Vf
Mf Mi
Calculate: VK2CrO4 = 0.2500 L 0.0100 mol 1.000 L = 0.0100 L 0.250 mol 1.00 L Slide 20 of 29
EXAMPLE 4-12 Determining the Limiting Reactant in a Reaction. Phosphorus trichloride, PCl3, is a commercially important compound used in the manufacture of pesticides, gasoline additives, and a number of other products. It is made by the direct combination of phosphorus and chlorine P4 (s) + 6 Cl2 (g) → 4 PCl3 (l)
What mass of PCl3 forms in the reaction of 125 g P4 with 323 g Cl2? Strategy: Compare the actual mole ratio to the required mole ratio. Slide 22 of 29
When actual yield = % yield the reaction is said to be quantitative. Side reactions reduce the percent yield. By-products are formed by side reactions.
Consecutive Reactions, Simultaneous Reactions and Overall Reactions Multistep synthesis is often unavoidable. Reactions carried out in sequence are called consecutive reactions. When substances react independently and at the same time the reaction is a simultaneous reaction. Slide 27 of 29
Overall Reactions and Intermediates The Overall Reaction is a chemical equation that expresses all the reactions occurring in a single overall equation. An intermediate is a substance produced in one step and consumed in another during a multistep synthesis.
